var styleElement = document.createElement('style');
styleElement.type = 'text/css';

// 添加自定义样式规则
var customStyle = '.md\\:max-w-3xl { max-width: 60rem !important; }';
if (styleElement.styleSheet) {
  // 兼容IE
  styleElement.styleSheet.cssText = customStyle;
} else {
  styleElement.appendChild(document.createTextNode(customStyle));
}

// 将新样式元素添加到页面头部
document.head.appendChild(styleElement);